Centrifugal clutch

ABSTRACT

A centrifugal clutch for coupling a drive shaft ( 11 ) to a driven member ( 13 ) at rotary speeds above a predetermined threshold, comprising: a centrifugal slider ( 15 ) with a massive enlargement ( 155 ) at one end and a first coupling formation ( 156 ) preferably at the other end; a frame ( 12 ) formed to carry the centrifugal slider on formations ( 121, 122, 123, 124 ) to constrain it to sliding motion between an extended radial position and a retracted radial position, and to fit on the drive shaft to be driven by it, with the shaft at right-angles to the axis of sliding motion of the frame: an output drive member mountable for free rotation on the drive shaft and formed for driving engagement with the driven member in use, and formed with a second coupling formation ( 131 ) which connects drivingly with the first only when the centrifugal slider is at its extended position; and means ( 16 ) preferably located wholly within a recess ( 151 ) in the centrifugal slider, for biasing the centrifugal slider toward its retracted position; whereby rotation of the centrifugal slider and frame causes the massive enlargement to pull the centrifugal slider radially from its retracted to its extended radial position to cause the first ( 155 ) and second coupling arrangements to interengage and thus to transmit rotary drive from the drive shaft to the driven member, but the biasing means causes disengagement when the rotation ceases, so as to decouple the drive shaft from the driven member.

This invention relates to a centrifugal clutch for coupling a driveshaft to a driven member. Centrifugal clutches are required for examplein latch arrangements for automotive doors, as disclosed in WO98/27301,where an electric drive motor has to be decoupled from the latch, whichis the driven member, to allow manual actuators to operate the latch, soas to avoid jamming particularly on power failure.

Centrifugal clutches are of course available in many different forms.The purpose of the present invention is to provide a centrifugal clutchwith the minimum number of component pieces, capable of easy assembly ata minimum cost. Such considerations are of prime importance inautomotive manufacture, where lightness in weight is also extremelyimportant.

Accordingly, the present invention provides a centrifugal clutch forcoupling a drive shaft to a driven member at rotary speeds above apredetermined threshold, comprising:

a centrifugal slider with a massive enlargement at one end and a firstcoupling formation;

a frame formed to carry the centrifugal slider on formations toconstrain it to sliding motion between an extended radial position and aretracted radial position, and to fit fixedly on the drive shaft to bedriven by it, with the shaft at right-angles to the axis of slidingmotion of the frame;

an output drive member mountable for free rotation on the drive shaftand formed for driving engagement with the driven member in use, andformed with a second coupling formation which connects drivingly withthe first only when the centrifugal slider is at its extended position;

and means, located wholly within a recess in the centrifugal slider, forbiasing the centrifugal slider towards its retracted position;

whereby rotation of the centrifugal slider and frame causes the massiveenlargement to pull the centrifugal slider radially from its retractedto its extended radial position to cause the first and second couplingarrangements to interengage and thus to transmit rotary drive from thedrive shaft to the driven member, but the biasing means causesdisengagement when the rotation ceases, so as to decouple the driveshaft from the driven member.

Recessing the biasing means allows the clutch to be extremely compact inthe axial direction, and by thus shortening the shaft the eccentricvibration of the output drive member is minimised, giving a stablestructure with long life.

Preferably, the first coupling formation is at the other end of thecentrifugal slider. This balances the impulsive forces upon engagementof the clutch. It is also advantageous that the coupling formation isthen at a substantially shorter radius than the massive enlargement, atthe point of engagement, because a larger impulsive force is then givenas a “kick” to the output drive, freeing any possible jam or excessivefriction.

The invention also provides a centrifugal clutch for coupling a driveshaft to a driven member at rotary speeds above a predeterminedthreshold, comprising:

a centrifugal slider with a massive enlargement at one end and a firstcoupling formation;

a frame formed to carry the centrifugal slider on formations toconstrain it to sliding motion between an extended radial position and aretracted radial position, and to fit fixedly on the drive shaft to bedriven by it, with the shaft at right-angles to the axis of slidingmotion of the frame;

an output drive member mountable for free rotation on the drive shaftand formed for driving engagement with the driven member in use, andformed with a second coupling formation which connects drivingly withthe first only when the centrifugal slider is at its extended position;

and a return spring acting on the centrifugal slider, and also bearingdirectly on the drive shaft in use, to bias the centrifugal slidertoward its retracted position;

whereby rotation of the centrifugal slider and frame causes the massiveenlargement to pull the centrifugal slider radially from its retractedto its extended radial position to cause the first and second couplingarrangements to interengage and thus to transmit rotary drive from thedrive shaft to the driven member, but the spring causes disengagementwhen the rotation ceases, so as to decouple the drive shaft from thedriven member.

With the spring bearing directly on the drive shaft, the arrangement isparticularly compact, with the minimum number of parts, and the simplestconstruction.

Further the invention provides a drive assembly comprising a drive shaftcoupled to the frame of a centrifugal clutch for coupling a drive shaftto a driven member at rotary speeds above a predetermined threshold,comprising:

a centrifugal slider with a massive enlargement at one end and a firstcoupling formation;

a frame formed to carry the centrifugal slider on formations toconstrain it to sliding motion between an extended radial position and aretracted radial position, and to fit fixedly on the drive shaft to bedriven by it, with the shaft at right-angles to the axis of slidingmotion of the frame;

an output drive member mountable for free rotation on the drive shaftand formed for driving engagement with the driven member in use, andformed with a second coupling formation which connects drivingly withthe first only when the centrifugal slider is at its extended position;

and means for biasing the centrifugal slider towards its retractedposition;

whereby rotation of the centrifugal slider and frame causes the massiveenlargement to pull the centrifugal slider radially from its retractedto its extended radial position to cause the first and second couplingarrangements to interengage and thus to transmit rotary drive from thedrive shaft to the driven member, but the biasing means causesdisengagement when the rotation ceases, so as to decouple the driveshaft from the driven member;

in which the drive shaft projects through the frame and the slider.

Extending the drive shaft through the whole clutch gives optimumstability, and ease of assembly.

Further, the invention provides a centrifugal clutch comprising drivingand driven coaxial members, the radially inner member having a sliderconstrained to slide between an extended radial position at which itengages a tooth on the radially outer member and drivingly couples themembers, and a retracted radial position at which it allows free rotaryrelative movement of the members;

a spring biasing the slider to its retracted position;

and a centrifugal crank lever, pivotally mounted on the radial innermember on an axis parallel to that of the members and spaced therefrom,having a massive enlargement at one end and pivotally connected to theslider at its other end such that rotation of the inner member causesthe massive enlargement to turn the crank lever to drive the slideragainst the spring bias to its extended position, but the spring causesthe slider to return to its retracted position when the clutch ceasesrotation.

A method of assembly of a drive arrangement, according to the invention,comprises pre-assembling a centrifugal clutch and then mounting thepre-assembled clutch over a drive shaft to effect a push fit;

the clutch comprising a centrifugal slider carried by a frame, a gearfor conveying rotary drive from the slider to an external driven memberin use, and a spring biasing the slider to a radial position when it isout of driving engagement with the gear, the slider being formed so asto slide under centrifugal action against the spring bias into drivingengagement with the gear when the clutch is rotated;

and the pre-assembled clutch having an axial opening, extending throughthe slider and the frame and axially into the gear, for receiving thedrive shaft.

By retaining the spring in the slider such that there is still anopening for the free axial passage of the motor drive shaft through it,it is possible to use a stronger spring than if it were necessary tocompress the spring during assembly. The use of a strong spring givesimproved efficiency in disengaging the clutch in use. By carrying thespring wholly within the slider, the arrangement can be made morecompact and its assembly can be simplified.

In order that the invention may be better understood, two embodimentswill now be described, by way of example only, with reference to theaccompanying diagrammatic drawings, in which:

FIG. 1 is an exploded perspective view from the motor drive side of acentrifugal clutch embodying the invention and part of an electric drivemotor;

FIG. 2 is a perspective view from the motor side of the assembledcentrifugal clutch of FIG. 1, with the centrifugal slider in itsretracted position;

FIG. 3 is a perspective view corresponding to FIG. 2 but with thecentrifugal slider in its extended position;

FIG. 4 is a perspective view from the driven member side of the assemblyof FIGS. 1 and 2;

FIGS. 5 and 6 are plan views of a centrifugal clutch according to asecond embodiment of the invention, with the centrifugal sliderrespectively extended and retracted.

A first embodiment will be described with reference to FIGS. 1 to 4.

A centrifugal clutch comprises a centrifugal slider 15, a spring 16, aframe 12 for connection to the drive shaft 11 of an electric motor 10,and an output drive member 13 which is a single toothed gear in thisexample.

The frame 12 is generally planar and rectangular, but is cut away at 126to accommodate part of the centrifugal slider 15. Four flanges 121, 122,123, 124 extending parallel to the plane of the frame 12 but spacedaxially from it, are arranged in a rectangle to guide the centrifugalslider 15 and to constrain it to sliding motion transverse to the axisof the motor spindle 11. An integral bush 125 projecting axially fromthe centre of the frame 12 coincides with an aperture through the frame,for accommodating the motor spindle 11. The motor spindle 11 is knurled(not shown) to provide rough projections for engagement against theinner surface of the bush 125, to form a push-fit when the clutchpre-assembly is pushed over the motor spindle 11.

The centrifugal slider 15 is also generally planar and rectangular, witha length similar to that of the frame 12, but narrower in width so as tobe accommodated within the guides 121 to 124. The slider 15 has amassive enlargement 155 at one end, projecting axially both above andbelow the plane of the slider. The radial exterior surface of theenlargement 155 is part-cylindrical, with its centre of curvature on theaxis. The slider 15 has an oval slot 152 at its centre, through whichthe motor spindle 11 extends when assembled, such as to allow the slider15 to slide radially between a retracted position as shown in FIG. 2 andan extended position as shown in FIG. 3.

The slider 15 is resiliently biased towards its retracted position asshown in FIG. 2 by means of a flat zig-zag steel spring 16 which isaccommodated wholly within a shallow recess 151 in the surface of theslider 15. The spring 16 fits into the recess, with one end abuttingagainst a pair of shoulders 153, 154 positioned about halfway along theopening 152, which still leaves an unobstructed opening equal to orslightly greater than the outer diameter of the spindle 11. This allowsfor automatic assembly of the clutch over the spindle 11, since it isnot necessary to compress further the spring 16 during insertion of theclutch pre-assembly 12, 13, 15 over the spindle 11.

The output drive member comprises, in this example, a single-toothedgear 13 formed integrally with a multiply-toothed gear 14. The singletooth 131 of the first gear 13 projects radially beyond the teeth of thesecond gear 14. An axial opening 132, broader than the outer diameter ofthe motor spindle 11, allows the gears 13 and 14 to be accommodated onthe spindle 11 but to rotate freely relative to the spindle 11. In use,the teeth on the second gear 14 convey motor drive to a driven member(not shown), when and only when the centrifugal slider 15 is in itsextended position shown in FIG. 3, at which the single tooth 131 engagesa tooth 156 (FIG. 4) on the centrifugal slider 15, projecting axiallyfrom the slider, away from the motor 10, and located at the end of theslider opposite to that of the massive enlargement 155.

The centrifugal clutch is assembled as follows. As shown in FIG. 1, thespring 16 is inserted into the recess 151, and the slider 15 is thenslotted into the frame 12 until the unobstructed opening 152 is inregister with the axial opening 125 in the frame 12. The spring is thusfully supported, and is carried wholly within the slider 15. Thepre-assembled three components 12, 15, 16 are then pushed axially overthe motor spindle 11 until a permanent push-fit is achieved, with partof the spindle 11 projecting from the clutch pre-assembly. The gears 13,14 are then mounted over the projecting spindle 11, as shown moreclearly in FIG. 4, and the combination of the motor, clutch and gear 14is then assembled into the door latch (or alternative receivingassembly) for engagement with the driven member, as appropriate. Thisassembly can be fully automated, using pick-and-place action, as it isparticularly simple and straight forward.

Whilst in this example the gears 13 and 14 are formed integrally, thisis not essential: they could be formed as separate components, forexample with a push-fit together.

In this example, the frame 12 and gears 13 and 14 are all plasticsinjection mouldings. The slider 15 can be of metal or plastic with ametal insert, to give it a sufficient mass for the centrifugal actionand sufficient smoothness of sliding operation.

An important feature of this example is that the tooth 156 for engagingthe gear pinion 13 is diametrically opposite the massive enlargement155. Thus the clutch coupling arrangement is separated from thecentrifugal mass arrangement, allowing for a more efficient powertransfer in operation, and greater simplicity in construction. In theclutch disclosed for example in WO95/24534 (Rockwell), the massiveenlargement functions itself as the coupling between the clutch halves,i.e. both are on the same radial side of the axis, and the coupling hasto take place at the same extreme radial position. With the preferredfeature, the coupling tooth 156 is diametrically opposite theenlargement 155, and the point of engagement of the tooth 156 and thegear tooth 131 is much closer to the axis, e.g. at half the radius ofthe enlargement 155. Thus the gear 13 can be smaller, and the powercoupling more efficient. Firstly, the spindle acts as a pivot, andcontributes to the dynamic stability of the clutch.

Secondly coupling takes place between facing surfaces of the gear tooth131 and the tooth 156 at a smaller radius than that of the massiveenlargement, e.g. at half the radius, which means a correspondinglygreater force on the opposed surfaces; this means the contact area canbe reduced, and this in turn reduces the likelihood of frictionalresistance preventing retraction of the slider. Thirdly, with thecoupling at a smaller radius, the gear 13, and the slider 15 and frame12, can be made sufficiently rigid to reduce substantially theirtendency to bend about the axis at the time of impact as the clutchengages, when there is an angular impulsive force.

A further novel feature of this arrangement is that the centrifugalslider 15 is spring biased against the motor spindle shaft 11, by directcontact between the spring 16 and the spindle 11. This allows for areduction in the number of component parts.

Typically, when compared with conventional centrifugal clutches, thenumber of component parts has been reduced from nine down to four. Thismakes a significant saving in material costs and in assembly costs andthe simplicity of the arrangement with reduced stresses gives it alonger life-cycle. Smaller housings may be used, leading to furthersavings, and greater adaptability to a variety of applications.

The invention also allows the motor drive spindle 11 to be shorter thanwith other conventional clutches, and this further assists in making thehousing smaller.

As an alternative to the use of a spring, it would be possible to use amagnetic bias between the slider 15 and the other components. This couldinvolve a permanent magnet acting on magnetic metallic parts of thecomponents, with the force of the magnet balancing the centrifugal forceat a predetermined speed of rotation.

An alternative arrangement, as a second embodiment of the invention, isshown in FIGS. 5 and 6.

A centrifugal clutch comprises a driving cylinder 512 and coaxial drivencylinder 514. The radially inner, driving cylinder 512 is fixed to themotor drive spindle 11, and has a slider 515 constrained by pins 26, 27and 30 on the inner cylinder to slide between an extended radialposition, shown in FIG. 5, at which it engages a tooth 513 on theradially outer cylinder and drivingly couples the cylinders, and aretracted radial position, shown in FIG. 6, at which it allows freerotary relative movement of the cylinders. A coil spring 22, mountedover a pin 29, has one end 25 coupled to the rim of the inner cylinder512, and the other end in engagement with a centrifugal crank lever 21which is mounted pivotally on the same pin 29 on the inner cylinder 512.The crank lever 21 has a massive enlargement 20 at one end, at the endof a long radius arm; the other arm of the crank lever, which is shorterto provide leverage and a mechanical advantage, is coupled pivotally at23 to the end of the centrifugal slider 24. Another pin 28 on the innercylinder acts as an end stop to limit the anti-clockwise rotation of thecrank lever 21 to the position shown in FIG. 5. At this limit position,the massive enlargement 20 does not interfere with the motion of thedriven coaxial cylinder 514.

In this example, although it is not essential, the slider 24 also has amassive enlargement 515 at its radially outer end, opposite to that ofthe pivot point 23. This assists in the centrifugal action.

In use, when the motor spindle 11 is stationary, the spring 22 drivesthe crank lever 21 clockwise as shown in FIGS. 5 and 6, towards theposition shown in FIG. 6, at which the slider 24 is retracted, and theclutch is disengaged. Once the motor speed has increased to a particularpredetermined speed, the centrifugal force on the massive enlargement515, combined with the centrifugal force on the massive enlargement 20,in combination overcome the spring bias, and drive the slider 24 to itsextended position as shown in FIG. 5 When the rotation decelerates onceagain, the spring will eventually overcome the centrifugal forces andreturn the slider to its retracted position, to disengage the clutch.

It will be understood that alternative mechanical arrangements arepossible for example to have a spring acting on the slider 24 insteadof, or in addition to, the spring 21 acting on the crank lever. Thecentre of mass of the slider 24, in the example given, is displacedradially a considerable distance from the axis of the motor spindle 11,because of the massive enlargement 515, but in some examples this maynot be necessary, depending upon the centrifugal force available fromthe enlargement 20 on the crank lever and the mechanical advantageprovided by the crank lever, together with an allowance made for thefrictional losses in the assembly.

The driving and driven “cylinders” need not be perfectly cylindrical,and could have less regular shapes, provided they are basically coaxial.

1. A centrifugal clutch for coupling a drive shaft to a driven member atrotary speeds above a pre-determined threshold, comprising: acentrifugal slider with a massive enlargement at one end and a firstcoupling formation; a frame formed to carry the centrifugal slider onformations to constrain it to sliding motion between an extended radialposition and a retracted radial position, and to fit fixedly on thedrive shaft to be driven by it, with the shaft at right-angles to theaxis of sliding motion of the frame; an output drive member mountablefor free rotation on the drive shaft and formed for driving engagementwith the driven member in use, and formed with a second couplingformation which connects drivingly with the first only when thecentrifugal slider is at its extended position; and a return springacting on the centrifugal slider, and also bearing directly on the driveshaft in use, to bias the centrifugal slider towards its retractedposition; whereby rotation of the centrifugal slider and frame causesthe massive enlargement to pull the centrifugal slider radially from itsretracted to its extended radial position to cause the first and secondcoupling arrangements to interengage and thus to transmit rotary drivefrom the drive shaft to the driven member, but the spring causesdisengagement when the rotation ceases, so as to decouple the driveshaft from the driven member.
 2. A centrifugal clutch according to claim1, in which the spring is flat.
 3. A centrifugal clutch according toclaim 2, in which the spring has a zig-zag shape.
 4. A centrifugalclutch according to claim 1, in which the frame is a plastics injectionmolding.
 5. A centrifugal clutch according to claim 1, or 4, in whichthe output drive member is a plastics injection molding.
 6. Acentrifugal clutch according to claim 1, in which the drive shaftprojects through the frame and the slider.
 7. A drive assemblycomprising a drive shaft coupled to the frame of a centrifugal clutchfor coupling a drive shaft to a driven member at rotary speeds above apredetermined threshold, comprising: a centrifugal slider with a massiveenlargement at one end and a first coupling formation; a frame formed tocarry the centrifugal slider on formations to constrain it to slidingmotion between an extended radial position and a retracted radialposition, and to fit fixedly on the drive shaft to be driven by it, withthe shaft at right-angles to the axis of sliding motion of the frame; anoutput drive member mountable for free rotation on the drive shaft andformed for driving engagement with the driven member in use, and formedwith a second coupling formation which connects drivingly with the firstonly when the centrifugal slider is at its extended position; and meansfor biasing the centrifugal slider towards its retracted position;whereby rotation of the centrifugal slider and frame causes the massiveenlargement to pull the centrifugal slider radially from its retractedto its extended radial position to cause the first and second couplingarrangements to interengage and thus to transmit rotary drive from thedrive shaft to the driven member, but the biasing means causesdisengagement when the rotation ceases, so as to decouple the driveshaft from the driven member; in which the drive shaft projects throughthe frame and the slider.
 8. A drive assembly according to claim 7, inwhich the frame is a plastics injection moulding.
 9. A drive assemblyaccording to claim 7 or 8, in which the output drive member is aplastics injection moulding.
 10. A centrifugal clutch comprising:driving and driven coaxial members, a radially inner member having aslider constrained to slide between an extended radial position at whichit engages a tooth on a radially outer member and drivingly couples themembers, and a retracted radial position at which it allows free rotaryrelative movement of the members; a spring biasing the slider to itsretracted position; and a centrifugal crank lever, pivotally mounted onthe radial inner member on an axis parallel to that of the members andspaced therefrom, having a massive enlargement at one end and pivotallyconnected to the slider at its other end such that rotation of the innermember causes the massive enlargement to turn the crank lever to drivethe slider against the spring bias to its extended position, but thespring causes the slider to return to its retracted position when theclutch ceases rotation.
 11. A centrifugal clutch according to claim 10,in which the driving and driven coaxial cylinders are plastics injectionmouldings.
 12. A centrifugal clutch according to claim 11, in which thecrank lever is also a plastics injection moulding.
 13. A motor drivearrangement, comprising an electric motor whose output shaft isconnected to a centrifugal clutch according to any one of claim 1, 7 or10.
 14. A method of assembly of a drive arrangement, comprising:pre-assembling a centrifugal clutch and then mounting the pre-assembledclutch over a drive shaft to effect a push fit; the clutch comprising acentrifugal slider carried by a frame, a gear for conveying rotary drivefrom the slider to an external driven member in use, and a spring whichabuts against the drive shaft and biases the slider to a radial positionwhen it is out of driving engagement with the gear, the slider beingformed so as to slide under centrifugal action against the spring biasinto driving engagement with the gear when the clutch is rotated; andthe pre-assembled clutch having an axial opening, extending through theslider and the frame and axially into the gear, for receiving the shaft.15. A method according to claim 14, in which the drive shaft is that ofa motor.
 16. A centrifugal clutch for coupling a drive shaft to a drivenmember at rotary speeds above a predetermined threshold, comprising: acentrifugal slider with a massive enlargement at one end and a firstcoupling formation; a frame formed to carry the centrifugal slider onformations to constrain it to sliding motion between an extended radialposition and a retracted radial position, and to fit fixedly on thedrive shaft to be driven by it, with the shaft at right-angles to theaxis of sliding motion of the frame; an output drive member mountablefor free rotation on the drive shaft and formed for drive engagementwith the driven member in use, and formed with a second couplingformation which connects drivingly with the first only when thecentrifugal slider is at its extended position; and means, locatedwholly within a recess in the centrifugal slider, for biasing thecentrifugal slider towards its retracted position; whereby rotation ofthe centrifugal slider and frame causes the massive enlargement to pullthe centrifugal slider radially from its retracted to its extendedradial position to cause the first and second coupling arrangements tointerengage and thus to transmit rotary drive from the drive shaft tothe driven member, but the biasing means causes disengagement when therotation causes, so as to decouple the drive shaft from the drivenmember.
 17. A method of assembly of a drive arrangement, comprisingpreassembling a centrifugal clutch and then mounting the pre-assembledclutch over a drive shaft to effect a push fit; the clutch comprising acentrifugal slider carried by a frame, a gear for conveying rotary drivefrom the slider to an external driven member in use, and a springbiasing the slider to a radial position when it is out of drivingengagement with the gear, the slider being formed so as to slide undercentrifugal action against the spring bias into driving engagement withthe gear when the clutch is rotated; and the pre-assembled clutch havingan axial opening, extending through the slider and the frame and axiallyinto the gear, for receiving the drive shaft.